Question

The molecular mass of Mohr’s salt, $FeS{{O}_{4}}{{(N{{H}_{4}})}_{3}}S{{O}_{4}}\cdot 6{{H}_{2}}O$ 392. Its equivalent mass is –
[A] 196
[B] 39.2
[C] 98.0
[D] 392

Answer 1

Hint:
Equivalent weight of a compound can be calculated from the atomic weight of the compound. To find the equivalent weight of Mohr salt, we can divide the given molecular mass by the change it undergoes in the oxidation state of iron in an oxidation-reduction reaction.

Complete step by step answer:
Equivalent weight is the mass of one equivalent I.e.it is the mass of a given substance which will combine or displace a fixed amount of another substance. Equivalent weight may be easily calculated from the molar mass.
Equivalent weight is given by the formula-

Equivalent weight= $\dfrac{Molecular\text{ }weight}{~valency\text{ }factor}$

Valency factor is the factor that helps us identify the number of valence electrons present in the valence shell of the given element.
As we know the formula of Mohr’s salt is $FeS{{O}_{4}}{{(N{{H}_{4}})}_{3}}S{{O}_{4}}\cdot 6{{H}_{2}}O$
We also know that it is a common reducing agent.
The Fe present in the Mohr salt is in +2 oxidation state. It oxidises by the removal of 1 electron I.e. it oxidises from $F{{e}^{2+}}$ to $F{{e}^{3+}}$. The oxidation state changes from +2 to +3. Therefore, it acts as a reducing agent. As we know, reducing agents oxidises themselves, we can write the reaction for its oxidation as-

                                     \[F{{e}^{2+}}\to F{{e}^{3+}}+{{e}^{-}}\]

As we can see, oxidation state of iron changes from +2 to +3 therefore, valency factor or n-factor of Mohr salt will be (3-2) =1
As it is given in the question, molecular mass of Mohr’s salt= 392

So, \[Equivalent\text{ }weight=\dfrac{Molecular\text{ }weight}{~valency\text{ }factor}=\dfrac{392}{1}\]

As we can see, the equivalent weight of Mohr’s salt is equal to its molecular weight for this reaction.

Therefore, option [D] 392 is the correct answer.

Note:
It is important to remember that equivalent weight is the gram molecular weight, it can be calculated in a number of ways depending on the type of reaction. But for compounds acting as oxidizing or reducing agents, it can be calculated by dividing the molecular mass of the given compound by the change in the number of positive or negative electrical charges during reduction or oxidation.